Vehicle power assisted steering systems typically use permanent, hardware-based systems to mechanically limit steering rack travel. These systems add weight and cost to the vehicle. Because a hardware-based system is permanent, the steering rack travel is fixed. In addition, different hardware can be required for each vehicle due to different wheel diameters and vehicle dimensions.
The recent development of electronically/electrically/electric power assisted steering (EPAS) provides several advantages over conventional power assisted systems. EPAS systems reduce weight and consume less power. Unlike hydraulic power assisted steering systems which continuously use power, the electronic motor used by an EPAS system is only used when needed during steering maneuvers. Due to lower power consumption, EPAS systems may prove more economical in use.
Existing power assisted steering systems typically use a single steering rack travel limit, i.e., the degree that the wheel may turn is fixed. The limiter is designed to prevent the tire from contacting the wheel well of the vehicle. To prevent the tire from contacting the wheel well using a single limiter, the limiter is positioned based on the most adverse conditions the vehicle may encounter. Conditions which limit the steering rack travel include suspension travel (e.g., jounce and rebound), speed of the vehicle, relative wheel speed, and lateral acceleration forces. Due to accommodation of these conditions, power assisted steering systems limit the vehicle to a larger than necessary turning radius because they are based on the most extreme possible operating conditions.
To increase driver satisfaction, it is desirable to reduce a vehicle's turning radius during low speed maneuvers. For example, when driving at low speed in a parking lot, a smaller turning radius is desirable.